Overlay offset measurement is implemented by an angle resolved scatterometry technology, such as the 4-cell measurement technology. In this arrangement, an illumination radiation is incident on a “grating on grating” target. The target scatters the illumination radiation to form a scattered radiation. A part of the scattered radiation undergoes diffraction by the target. The resulting scattered radiation pattern consists of several diffracted orders, according to the grating equation. This scattered radiation is then collected and analyzed, where the spatial and/or angular distribution of the diffracted light is used to estimate the overlay offset between the layers of the “grating over grating” target.
In typical lithography processes, the optimal direction of the grating is dictated by the lithography illumination pattern. Typically a dipole or quadrapole illumination is used to minimize the critical dimension of the structures, and the grating direction is fixed by the choice of illumination.
U.S. Patent Publication No. 2011/0194092 discloses rotating the actual target grating, which raises a large number of applicative problems. U.S. Patent Publication No. 2011/0194092 maintains the target periodicity itself and does not maintain the direction target periodicity nor modify field stop boundaries and/or target boundaries.